"To some, it may seem like an ideal relationship, less stressful than an affair, longer lived than a fling or that elusive one-night stand. You can even sit around in your sweats and watch “Friends” reruns together, feeling vaguely reassured.

Yet relationships in which close friends begin having sex come with their own brand of awkwardness, according to the first study to explore the dynamics of such pairs, often called friends with benefits, or F.W.B..

The relationships tend to have little romantic passion, but stir the same fears that stalk lovers: namely, that one person will fall harder than the other.

Paradoxically, and perhaps predictably, the study suggests, these physical friendships often occlude one of the emotional arteries of real friendship, openness. Friends who could once talk about anything now have an unstated taboo topic — the relationship itself. In every conversation, there is innuendo; in every room, an elephant.

The research, conducted among Michigan State University students, confirmed previous findings that most college students report having had at least one such relationship. Although that is undoubtedly true of many couples throughout history, “friends with benefits” have become a cultural signature of today’s college and postcollege experience.

“The study really adds to the little we know about these relationships,” said Paul Mongeau, a professor of communications at Arizona State University who was not involved in the research. “One of the most interesting things I get from it,” he said, “is this sense that people in these relationships are afraid to develop feelings for the other person, because those feelings might be unreciprocated.”(Complete NYTimes.com article)